Move away from relying on ExecutionContext in Grains dependencies and OnActivateAsync()

[einari]

Today we are relying on the execution context to be set and correct. This turns out to be hard with the Orleans call model, besides our grains now have all the information on their compound keys telling it which microservice & tenant they belong to.

Any underlying storage mechanisms we depend on should instead therefor have a provider that can provide it for the known microservice and/or tenant.

Should be looked at when doing #264

[einari]

By doing this we should also remove the call filters for execution context.
This should improve memory utilization and performance. We're seeing the Orleans serializer kicked in for the execution context all the time, which is completely not needed.

[einari]

One thing to remember is logging, making sure we get enough metadata on log messages to identity MicroserviceId, TenantId, CorrelationId, CausationId and CausedBy. We're today doing this through a custom Serilog extension which is linked to the execution context.